Most electronic circuits are built into some kind of a box that is metal. The metal chassis is usually "GROUND" or also called "negative" of the power supply. The metal chassis is also connected to the house mains (AC voltage) to the GROUND terminal which goes into the electrical box and finally out the the big transformer on the telephone pole. The transformer on the pole has about 5000 volts coming into it... then 220volts CENTER-TAPPED comes out. You can get 110 volts out of it by connecting to the center-tap and either of the other output leads from the transformer. The center tap is also connected to a metal rod hammered into the ground. This is also called a GROUND connection and it's purpose is to give lightning-strikes a path to EARTH ground. My answer relates to the USA's electrical grid. European electric is often ALL 220 volts, they dont use 110 volts.

That's not entirely true. The metal chassis is indeed connected to earth ground on many appliances. This is done in case of a fault inside the case, in which a "hot" 120V wire were to touch the case. Without a ground, someone could touch the case and receive a nasty shock. Grounding the chassis causes the circuit breaker to trip, protecting the user (and the equipment as well.)

You are mixing up neutral and ground in the second half of your explanation. The transformer is indeed tapped to get 120V, but the two wires used are "hot" and "neutral." Ground is a third, separate wire that has its own path back to the fuse panel. During normal conditions neutral will be basically at the same potential as ground (go ahead and measure them with a multimeter). But, they should not be mixed up.

The term "ground" is also used somewhat interchangeably. There's "earth ground" which is the physical connection to earth, and there's "ground reference" which is usually the "0V" reference voltage. Sometimes they are connected together, and sometimes they are not. The choice in whether to connect them can have positive or negative implications - for instance, it can cause "ground loop" hum in audio equipment. But it can also help make sure that connected equipment is all at the same potential (to prevent potential over-voltages).

On some schematics you'll also see separate analog and digital grounds, a practice that is debated amongst electrical engineers about whether it's worthwhile doing at all.

A good designer will be careful to specify on a schematic diagram or wiring diagram which ground is being used, and will connect them carefully to make sure that everything operates safely and without glitches.

I was referring to ground planes. Some designers will use separate analog and digital ground planes, but others say that it doesn't make a lick of difference (and may make things worse, in some cases).

And yes, star ground for the win - that's what a ground plane is, a star with infinite points. ;)

Electronic ground is a true circuit ground that leads directly to the power supplies ground. A common ground is similar just think instead of a circuit board, a vehicle electrical system ground.An Earth Ground is basically like a Home Electrical system in which the Ground is quite literally Grounded to the earth through the supply panel.